Tag Archives: anchor cage

Readers' questions: may I use an anchor cage somewhere else?

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I’ve just received this question from a reader. As I believe it’s an interesting topic I’ve decided to answer with a post instead of an email or in the comment section.

“I'm custom broker and I have to classify under HS Code an anchor cage. I have consulted to Classification Office of Argentine Customs Service and they ask me if the anchor cages are designed to be used exclusively in the construction of wind generators, or if they could be used in other constructions, for instance an antenna tower.
I would appreciate if you could help me on this matter.
Kind regards.”

The answer is no – they can’t  be used somewhere else.

There are several applications for foundation cages: power transmission pylons, light poles, mobile phone antennas and other type of towers.

However, anchor cage are dimensioned to fit a specific type of tower. For instance, different wind turbine models have different anchor cages (both the number of bolts and their diameter might vary). You can’t take a generic anchor cage and put it below another tower: the number of bolts, diameter of the tower and size of the bolt would not match.

Some years ago there has been a famous mistake in a wind farm in Brazil – the wrong anchor cages have been shipped (and embedded in the concrete of the foundations). The mismatch between tower bottom and anchor cage was millimetric, so the installation crew tried to install the towers for hours before discovering the mistake. It was the anchor cage for a different model of tower for that specific wind turbine model.

It’s interesting to note that at least a wind turbine manufacturer offer a range of anchor cages with different bolt lengths compatible with a specific wind turbine model. This allow for a greater customization of the foundation and savings in material.

Anchor Cage assembly

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Here you have several picture of an anchor cage we assembled a few days ago.

The anchor cage is a system conceived to transfer the loads to the foundation more effectively, anchoring the tower to the foundation.

It is made of two ring-shaped steel plates, an anchor plate and a load distribution plate, secured by anchor bolts.

Bolts are tensioned to a standard value, or sometime to different values depending on the calculation made for every foundation.

This system has been used for many years – I think that one of the first manufacturer to use it was Nordex in 2000.

As you can see we received the bolts and the distance pipes in several wood boxes, together with nuts and washers – all manufactured by Cooper and Turner in the UK.

Other items we received where the template to help the workers in the mounting operations, the foam to protect the top of the bolts during grouting and the legs of the cage.

The assembly operation take several hours: the two halves are assembled separately and than joined together below using a fastener and above using a fishplate (there is a picture with the detail of the operation).

There is a tolerance of a few millimeters, so the operation can be time consuming as several manoeuvres with the crane are necessary. We double checked each time using a laser level.

Anchor cage foundations

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Anchor cage foundations are an alternative to the embedded ring and they will be a de facto standard in the future.

Basically an anchor cage is a set of bolts, kept together by an inferior and superior steel rings. It normally arrives disassembled to the site, and it is mounted by workers in a few hours.

The main advantage is a better transmission of loads to the concrete: sometimes a separation of the embedded ring from the concrete is observed, normally leading to movements of the tower and serious stability problems.

The first steps of anchor cage foundation are identical to the standard foundation: a hole is excavated into the soil with the dimensions indicated in the constructive project, and the bottom is prepared with about 20 cm of blinding concrete.

After the anchor cage is positioned in the middle. It can be assembled right in the foundation hole or somewhere nearby and than it’s moved using a small crane.

When it is in the final position, horizontality of the cage is checked and adjusted as needed (it rest on small feet). Then, reinforcement bars are placed: first radial bars, than concentric bars, shear bars and finally the superior group of bars.

When all bars are ready, concrete is poured and vibrated and the foundation is covered by soil.

Finally, the bottom section of the tower is guided into position and washers and nuts are positioned and pretensioned.

Final tensioning will be applied only when grouting is poured between the tower and the anchor cage. Grouting is a complicated operation that will be described in detail in another post.